Adapting scan-bist architectures for low power operation

ABSTRACT

A Scan-BIST architecture is adapted into a low power Scan-BIST architecture. A generator  102 , compactor  106 , and controller  110  remain the same as in the known art. The changes between the known art Scan-BIST architecture and the low power Scan-BIST architecture involve modification of the known scan path into scan path  502 , to insert scan paths A  506 , B  508  and C  510 , and the insertion of an adaptor circuit  504  in the control path  114  between controller  110  and scan path  502.

This application is a divisional of application Ser. No. 13/565,128,filed Aug. 2, 2012, currently pending;

Which was a divisional of application Ser. No. 13/184,077, filed Jul.15, 2012, now U.S. Pat. No. 8,261,144, granted Sep. 4, 2012;

Which was a divisional of application Ser. No. 13/043,778, filed Mar. 9,2011, now U.S. Pat. No. 8,015,466, granted Sep. 6, 2011;

Which was a divisional of application Ser. No. 12/780,410, filed May 14,2010, now U.S. Pat. No. 7,925,945, granted Apr. 12, 2011;

Which was a divisional of application Ser. No. 12/406,348, filed Mar.18, 2009, now U.S. Pat. No. 7,747,919, granted Jun. 29, 2010;

Which was a divisional of application Ser. No. 11/278,064, filed Mar.30, 2006, now U.S. Pat. No. 7,526,695, granted Apr. 28, 2009;

Which was a divisional of application Ser. No. 10/886,206, filed Jul. 6,2004, now U.S. Pat. No. 7,051,257, granted May 23, 2006;

Which was a divisional of application Ser. No. 09/803,608, filed Mar. 9,2001, now U.S. Pat. No. 6,763,488, granted Jul. 13, 2004;

Which claimed priority from Provisional Application 60/188,109, filedMar. 9, 2000.

The disclosure relates to and incorporates by reference U.S. Pat. No.6,519,729, issued Feb. 11, 2003, and U.S. Pat. No. 6,769,080, issuedJul. 27, 2004.

BACKGROUND OF THE DISCLOSURE

1. Field of the Disclosure

Scan-BIST architectures are commonly used to test digital circuitry inintegrated circuits. The present disclosure describes a method ofadapting conventional Scan-BIST architectures into low power Scan-BISTarchitectures.

2. Description of the Related Art

FIG. 1 illustrates a conventional Scan-BIST architecture that a circuit100 can be configured into during test. In the normal functionalconfiguration, circuit 100 may be a functional sub-circuit within IC,but in test configuration it appears as shown in FIG. 1. The Scan-BISTarchitecture is typically realized within a sub-circuit of an IC, suchas an intellectual property core DSP or CPU sub-circuit. The Scan-BISTarchitecture includes a generator circuit 102, compactor circuit 106,scan path circuit 104, logic circuitry to be tested 108, and controllercircuit 110. Generator 102 operates to produce and output serial teststimulus patterns to scan path 104 via path 118. Compactor 106 operatesto input and compress serial test response patterns from scan path 104via path 120. Scan path 104 operates, in addition to its serial inputand output modes, to output parallel test stimulus patterns to logic 108via path 122, and input parallel response patterns from logic 108 viapath 124. Controller 110 operates to produce and output the controlrequired to operate generator 102 via path 112, scan path 104 via path114, and compactor 106 via path 116. Generator 102 may be designed usingany suitable type of circuit for producing stimulus patterns, such aslinear feedback shift registers. Compactor 106 may be designed using anysuitable type of circuit for compacting response patterns intosignatures, such as signature analysis registers. Controller 110 may bedesigned using any suitable type of controller or state machine designedto autonomously operate generator 102, scan path 104, and compactor 106during test.

The circuit of FIG. 1 may be configured into the illustrated Scan-BISTarchitecture and enabled to start a test operation in response to avariety of methods, including; (1) in response to power up of thecircuit, (2) in response to manipulation of external inputs to thecircuit, or (3) in response to data loaded into a register, such as theIEEE 1149.1 TAP instruction register.

FIG. 2 illustrates an example of a conventional scan cell that could beused in scan path 104. (Note: The optional scan cell multiplexer 218 andconnection paths 220 and 224, shown in dotted line, will not bediscussed at this time, but will be discussed later in regard to FIGS. 7and 8.) The scan cell consists of a D-FF 204 and a multiplexer 202.During normal configuration of the circuit 100, multiplexer 202 and D-FF204 receive control inputs SCANENA 210 and SCANCK 212 to input andoutput functional data to logic 108 via paths 206 and 216, respectively.In the normal configuration, the SCANCK to D-FF 204 is typically afunctional clock, and the SCANENA signal is set such that the D-FFalways clocks in functional data from logic 108 via path 206. During thetest configuration of FIG. 2, multiplexer 202 and D-FF 204 receivecontrol inputs SCANENA 210 and SCANCK 212 to capture test response datafrom logic 108 via path 206, shift data from scan input path 208 to scanoutput path 214, and apply test stimulus data to logic 108 via path 216.In the test configuration, the SCANCK to D-FF 204 is the test clock andthe SCANENA signal is operated to allow capturing of response data fromlogic 108 and shifting of data from scan input 208 to scan output 214.During test configuration, SCANENA is controlled by controller 110.SCANCK may also be controlled by the controller, or it may be controlledby another source, for example the functional clock source. For thepurpose of simplifying the operational description, it will be assumedthat the SCANCK is controlled by the controller.

The scan inputs 208 and scan outputs 214 of multiple scan cells areconnected to form the serial scan path 104. The stimulus path 216 andresponse path 206 of multiple scan cells in scan path 104 form thestimulus bussing path 122 and response bussing path 124, respectively,between scan path 104 and logic 108. From this scan cell description, itis seen that the D-FF is shared between being used in the normalfunctional configuration and the test configuration. During scanoperations through scan path 104, the stimulus outputs 216 from eachscan cell ripple, since the stimulus 216 path is connected to the scanoutput path 214. This ripple causes all the inputs to logic 108 toactively change state during scan operations. Rippling the inputs tologic 108 causes power to be consumed by the interconnect and gatingcapacitance in logic 108.

FIG. 3 illustrates a simplified example of the operation 300 ofcontroller 110 during test. Initially the controller will be in an idle302 or non-operational state. In response to a start test operationinput, for example using one of the methods mentioned above, thecontroller transitions from the idle state to the operate state 304. Inthe operate state, the controller issues control to the generator, scanpath, and compactor. In response to the control, the generator beginsproducing stimulus data to the scan path, the scan path begins acceptingthe stimulus data and outputting response data, and the compactor beginsinputting and compressing the response data from the scan path. Thecontroller remains in the operate state until the scan path has beenfilled with stimulus data and emptied of response data. From the operatestate, the controller passes through the capture state 306 to loadresponse data from the logic 108, then re-enters the operate state.Since the initial response data from the scan path may be unknown,unless for example the scan path is initialized at the beginning of thetest, the response data input to the compactor may be delayed or maskedoff until after the controller has passed through the capture state 206a first time. The process of entering the operate state to load stimulusinto the scan path and empty response from the scan path, then passingthrough the capture state to load new response data repeats until theend of test. At end of test the controller re-enters the idle state.Upon re-entering the idle state, the controller may output an end oftest (EOT) signal 111 to indicate test completion. The compactor may bedesigned to include an expected response signature value that iscompared against the signature obtained from the test. If so, thecompactor will typically output a PASS/FAIL signal 117 at end of test toindicate whether the signature taken matched the expected signature. Theuse of EOT and PASS/FAIL signals are assumed in subsequent Figures, butwill not be shown.

FIG. 4 illustrates a timing example of how controller 110 outputsSCANENA and SCANCK signals to scan path 104 during scan operations. Inthis example, a high to low transition on SCANENA, at time 406, incombination with SCANCKs occurring during time interval 402, causesstimulus data from generator 102 to be input to the scan path whileresponse data is output to compactor 106. A low to high transition onSCANENA, at time 408, in combination with a SCANCK at time 404, causesresponse data from logic 108 to be loaded into the scan path. Timeinterval 402 relates to operate state 304 and time interval 404 relatesto capture state 306 of FIG. 3. As seen in the timing and operationdiagrams of FIGS. 3 and 4, the time interval sequences 404 (i.e. state306) and 402 (i.e. state 304) cycle a sufficient number of times duringtest to input all stimulus to and obtain all response from logic 108.

From the Scan-BIST architecture described in regard to FIGS. 1, 2, 3,and 4 it is seen that the stimulus 122 outputs ripple the inputs tologic 108 as data shifts through the scan path 104 during scanoperations. Rippling the inputs of logic 108 causes simultaneouscharging and discharging of capacitance's associated with theinterconnects and gates of logic 108. For example, each scan cellstimulus output 216 to logic 108 charges and discharges a certain amountof capacitance within logic 108 at a frequency related to the data bitsbeing scanned through the scan cell. While each scan cell stimulusoutput may only be directly input to a few gates within logic 108, eachof the gates have outputs that fanout to inputs of other gates, and theoutputs of the other gates again fanout to inputs of still furthergates, and so on. Thus a transition on the stimulus output of a singlescan cell may initiate hundreds of transitions within logic 108 as aresult of the signal transition fanout.

The individual power (Pi) consumed by the rippling of a given scan celloutput 216 can be approximated by CV²F, where C is the capacitance beingcharged or discharged by the scan cell output (i.e. the capacitance ofthe above mentioned signal transition fanout), V is the switchingvoltage level, and F is the switching frequency of the scan cell output.The total power (Pt) consumed by simultaneously scanning all the scancells in scan path 104 is approximately the sum of the individual scancell powers, i.e. Pt=Pi₁+Pi₂+ . . . Pi_(N). The total power consumed bycircuit 100 when it is configured into the Scan-BIST architecture ofFIG. 1 can exceed the power consumed by circuit 100 when it isconfigured into its normal functional mode. This can be understood fromthe fact that, during normal functional mode of circuit 100, not all theD-FFs 204 simultaneously operate, as they do during scan operationsoccurring during the above described Scan-BIST test operation. Further,if an IC contained multiple circuits 100, the test of the IC may requiretesting each circuit 100 individually due to the above described testpower consumption restriction. This lengthens the test time of the IC,which increased the cost to manufacture the IC. This also lengthens thepowerup-self-test time of ICs in portable, battery operated systems.

A first known method of reducing power consumption during test operationis to insert blocking circuitry, such as a gate, into the stimulus paths216 of each scan cell, such that during scan operations the inputs tologic 108 are blocked from the effect of the scan ripple. The problemwith the first method is that it adds an undesirable delay (i.e. theblocking circuit delay) in the stimulus paths 216 between D-FFs 204 andlogic 108. This delay can negatively effect the performance of circuit100 when it is configured into its normal functional mode. A secondknown method is to reduce the scan clock rate, such that the ripplefrequency (F) is reduced. The problem with the second method is that itincreases the test time since scan operations are performed at thereduced scan clock rate.

Today, there are a number of test synthesis vendor tools that cansynthesize and insert Scan-BIST architectures into ICs, similar instructure to the Scan-BIST architecture shown in FIG. 1. The use of such“push-button” Scan-BIST insertion tools is an attractive alternative tocustomized Scan-BIST designs since it is an automated process. As willbe described, the present disclosure provides a method of adapting thesesynthesized Scan-BIST architectures such that they may operate in adesired low power mode. The process of adapting Scan-BIST architecturesfor low power operation is also easily automated.

BRIEF SUMMARY OF THE DISCLOSURE

Scan-BIST architectures are commonly used to test digital circuitry inintegrated circuits. The present disclosure describes a method ofadapting conventional Scan-BIST architectures into low power Scan-BISTarchitectures. The low power Scan-BIST architecture maintains the testtime of Scan-BIST architectures, while requiring significantly lessoperational power than conventional Scan-BIST architectures. The lowpower Scan-BIST architecture is advantageous to IC/die manufacturerssince it allows a larger number of circuits (such as DSP or CPU corecircuits) embedded in an IC/die to be tested in parallel withoutconsuming too much power within the IC/die. It is also advantageous todesigners of portable, battery operated systems, like wirelesstelephones, since ICs in the systems can be powerup-self-tested by thelow power Scan-BIST architecture using only a fraction of the storedbattery energy required by conventional scan-BIST architectures.

The present disclosure described below provides a method of adaptingsynthesized Scan-BIST architectures to achieve a low power mode ofoperation. The process of adapting Scan-BIST architectures for low poweroperation is achieved without having to modify the above mentionedsynthesized controller 110, generator 102, or compactor 106. Also, theprocess of adapting Scan-BIST architectures for low power operation isachieved without the aforementioned problems of; (1) having to insertblocking circuitry in the stimulus paths which adds signal delays, and(2) having to decrease the scan clock rate which increases test time.

A generator 102, compactor 106, and controller 110 remain the same as inthe known art. The changes between the known art Scan-BIST architectureand the low power Scan-BIST architecture involve modification of theknown scan path into a modified scan path, to insert scan paths A, B andC, and the insertion of an adaptor circuit in the control path 114between controller 110 and the scan path.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a block diagram of a SCAN-BIST circuit having a single scanpath.

FIG. 2 is a block diagram of a scan cell.

FIG. 3 is a flow diagram of the operation of the circuit of FIG. 1.

FIG. 4 is a timing diagram of the operation of the circuit of FIG. 1.

FIG. 5 is a block diagram of a SCAN-BIST circuit having a scan patharranged according to the present disclosure.

FIG. 6 is a flow diagram of the operation of the circuit of FIG. 5.

FIG. 7 is a block diagram of the adaptor of FIG. 5.

FIG. 8 is a timing diagram for the operation of the adaptor of FIG. 7.

FIG. 9 is a block diagram of the scan paths arranged according to thepresent disclosure.

FIG. 10 is a block diagram of a SCAN-BIST circuit using a conventionalparallel scan architecture.

FIG. 11 is a flow chart for the operation of the parallel scan path ofFIG. 10.

FIG. 12 is a block diagram of a SCAN-BIST parallel scan path arrangedaccording to the present disclosure.

FIG. 13 is a flow chart of the operation of the circuit of FIG. 12.

FIG. 14 is a block diagram of another SCAN-BIST parallel scan pathcircuit with the adaptor incorporated in the low cost controller.

FIG. 15 is a flow chart of the operation of the circuit of FIG. 14.

FIG. 16 is a block diagram of the circuit of FIG. 14 according to thepresent disclosure.

FIG. 17 is a flow chart of the operation of the circuit of FIG. 16.

DETAILED DESCRIPTION OF THE DISCLOSURE

FIG. 5 illustrates the Scan-BIST architecture of FIG. 1 after it hasbeen adapted into the low power Scan-BIST architecture of the presentdisclosure. In FIG. 5, it is seen that the generator 102, compactor 106,and controller 110 remain the same as in FIG. 1. The changes between theFIG. 1 Scan-BIST architecture and the FIG. 5 low power Scan-BISTarchitecture involve modification of scan path 104 into scan path 502,and the insertion of an adaptor circuit 504 in the control path 114between controller 110 and scan path 502.

Adapting scan path 104 into scan path 502 involves reorganizing scanpath 104 from being a single scan path containing all the scan cells(M), into a scan path having a desired number of selectable separatescan paths. In FIG. 5, scan path 502 is shown after having beenreorganized into three separate scan paths A, B, and C 506-510. It isassumed at this point in the description that the number of scan cells(M) in scan path 104 is divisible by three such that each of the threeseparate scan paths A, B, and C contains an equal number of scan cells(M/3). The case where scan path 104 contains a number of scan cells (M)which, when divided by the number of desired separate scan paths, doesnot produce an equal number of scan cells in each separate scan pathwill be discussed later in regard to FIG. 9.

Scan paths A, B, and C are configured as follows inside scan path 502.The serial input of each scan path A, B, and C is commonly connected tothe generator 102 via connection 118. The serial output of scan path Ais connected to the input of a 3-state buffer 512, the serial output ofscan path B is connected to the input of a 3-state buffer 514, and theserial output of scan path C is connected to the input of a 3-statebuffer 516. The outputs of the 3-state buffers 512-516 are commonlyconnected to compactor 106 via connection 120. Scan paths A, B, and Ceach output an equal number of parallel stimulus inputs 526, 530, 534 tologic 108, and each input an equal number of parallel response outputs524, 528, 532 from logic 108. The number of stimulus output signals tologic 108 in FIGS. 1 and 5 is the same. The number of response inputsignals from logic 108 in FIGS. 1 and 5 is the same. Scan path A andbuffer 512 receive control input from adaptor 504 via bus 518, scan pathB and buffer 514 receive control input from adaptor 504 via bus 520, andscan path C and buffer 516 receive control input from adaptor 504 viabus 522.

Adaptor 504 is connected to scan paths A,B,C via busses 518-522 and tocontroller 110 via bus 114. The purpose of the adaptor is to interceptthe scan control output 114 from controller 110 and translate it into asequence of separate scan control outputs 518-522 to scan paths A, B,and C, respectively. Each of the separate scan control outputs 518-522are used to operate one of the scan paths A, B, and C.

FIG. 6 illustrates a simplified example of the combined operation 600 ofthe controller 110 and adaptor 504 during test. The operation ofcontroller 110 is the same as previously described in regard to FIG. 3.When the controller transitions to the operate state 304, it beginsoutputting control to the generator 102, adaptor 504, and compactor 106.The generator and compactor responds to the control input as previouslydescribed in regard to FIGS. 1 and 3. The adaptor responds to thecontrol input by translating it into a sequence of separate controloutputs 518, 520, and 522 to scan paths A, B, and C. As indicated inadaptor operation block 602, the adaptor first responds to control 114during adaptor operation state 604 to output control 518, which enablesbuffer 512 and operates scan path A to input stimulus data fromgenerator 102 and output response data to compactor 106. After scan pathA is filled with stimulus and emptied of response, adaptor 504 respondsto control 114 during operation state 606 to output control 520, whichenables buffer 514 and operates scan path B to input stimulus data fromgenerator 102 and output response data to compactor 106. After scan pathB is filled with stimulus and emptied of response, adaptor 504 respondsto control 114 during operation state 608 to output control 522, whichenables buffer 516 and operates scan path C to input stimulus data fromgenerator 102 and output response data to compactor 106. After scanpaths A, B, and C have been filled and emptied, the controller 110transitions from the operate state 304, through the capture state 306,and back to the operate state 304. During this transition, the adaptoris idle during the capture state 306, but resumes its scan controlsequencing operation when the operate state 304 is re-entered. Thisprocess of sequentially scanning scan paths A, B, and C, then performinga capture operation to load response data repeats until the test hasbeen performed and controller 110 enters the idle state 302.

During the sequencing of the operation states 604-608, only one of thebuffers 512-516 are enabled at a time to output response data tocompactor 106. Also, the sequencing of the adaptor operation states604-608 occurs in a seamless manner such that the stimulus data from thegenerator 102 is input to scan path 502 as it was input to scan path104, and the response data to compactor 106 is output from scan path 502as it was output from scan path 104. To the controller, generator, andcompactor, the behavior of the scan path 502 and adaptor 504 combinationis indistinguishable from the behavior of the scan path 104 in FIG. 1.Thus the test time of the logic 108 in FIG. 5 is the same as the testtime of logic 108 in FIG. 1.

From the above description, it is seen that only a subset (i.e. subset A526, B 530, or C 534) of the stimulus input bus 122 to logic 108 isallowed to ripple at any given time during the adaptor operated scanoperation of FIGS. 5 and 6. In contrast, the entire stimulus input bus122 to logic 108 ripples during the controller operated scan operationof FIGS. 1 and 3. Since, using the present disclosure, only a subset ofthe stimulus inputs to logic 108 are allowed to ripple at any one time,less of the aforementioned interconnect and gating capacitance of logic108 is simultaneously charged and discharged during scan operations. Byreducing the amount of logic 108 capacitance being simultaneouslycharged and discharged during scan operations, the power consumed bylogic 108 is advantageously reduced by the present disclosure.

Example Adaptor Circuit

FIG. 7 illustrates an example adaptor circuit 504 implementation.Adaptor 504 inputs the SCANCK 212 and SCANENA 210 signals fromcontroller 110, via bus 114. Adaptor 504 outputs SCANCK-A signal 712,SCANCK-B signal 714, SCANCK-C signal 716, ENABUF-A signal 718, ENABUF-Bsignal 720, ENABUF-C signal 722, and the SCANENA signal 210. The SCANENAsignal 210 is connected to all scan cell 200 multiplexers 202 as shownin FIG. 2. The SCANCK-A signal 712 is connected, in substitution ofSCANCK signal 212, to all scan cell 200 D-FF 204 clock inputs of scanpath A. The SCANCK-B signal 714 is connected, in substitution of SCANCKsignal 212, to all scan cell 200 D-FF 204 clock inputs of scan path B.The SCANCK-C signal 716 is connected, in substitution of SCANCK signal212, to all scan cell 200 D-FF 204 clock inputs of scan path C. TheENABUF-A signal 718 is connected to the enable input of buffer 512. TheENABUF-B signal 720 is connected to the enable input of buffer 514. TheENABUF-C signal 722 is connected to the enable input of buffer 516.

Adaptor 504 includes a state machine 702, counter 704, and gates706-710. During functional mode of circuit 500, SCANENA is high asindicated at time 810 in the adaptor timing diagram of FIG. 8. WhileSCANENA is high, state machine 702 outputs control signals 724-728 thatenable SCANCK to pass through gates 706-710 to functionally clock allD-FFs 204 of scan paths A, B, and C, via SCANCK-A, SCANCK-B, andSCANCK-C. In this example, the SCANCK is assumed to be the functionalclock during the functional mode of circuit 500, and the test clockduring test mode of circuit 500. While SCANENA is high, state machine702 outputs control signals 718-722 to disable buffers 512-516. The scanoperation mode is entered by SCANENA going low as indicated at time 812in FIG. 8. SCANENA goes low when controller 110 transitions from theidle state 302 to the operate state 304 as seen in FIG. 6.

At the beginning of the scan operation mode, the state machineinitializes counter 704 via control (CTL) signals 730 and disables scanaccess to scan paths B and C by disabling SCANCK gates 708 and 710 viasignals 726 and 728, and enables scan access to scan path A by; (1)enabling SCANCK gate 706 via signal 724, and (2) enabling buffer 512 viasignal 718. Scan access of scan path A occurs over time interval 802 ofFIG. 8. During time interval 802, scan path A is accessed to loadstimulus data from generator 102 and unload response to compactor 106.While scan path A is being accessed, the state machine operates counter704 via control signals 730 to determine the number (M/3) of SCANCK-A'sto output to scan path A. When the counter reaches a count, indicativeof scan path A receiving the correct number (M/3) SCANCK-A inputs, itoutputs a first count complete 1 (CC1) signal 732 to state machine 702.

In response to the first CC1 signal, the state machine initializescounter 704 via control signals 730 and disables scan access to scanpath A and C, and enables scan access to scan path B over time interval804. The state machine enables scan access to scan path B by; (1)enabling SCANCK gate 708 via signal 726, and (2) enabling buffer 514 viasignal 720. While scan path B is being accessed, the state machineoperates counter 704 via control signals 730 to determine the number ofSCANCK-B's to output to scan path B. When the counter reaches a count,indicative of scan path B receiving the correct number (M/3) SCANCK-Binputs, it outputs a second count complete 1 (CC1) signal 732 to statemachine 702.

In response to the second CC1 signal, the state machine initializescounter 704 via control signals 730 and disables scan access to scanpath A and B, and enables scan access to scan path C over time interval806. The state machine enables scan access to scan path C by; (1)enabling SCANCK gate 710 via signal 728, and (2) enabling buffer 516 viasignal 722. While scan path C is being accessed, the state machineoperates counter 704 via control signals 730 to determine the number ofSCANCK-C's to output to scan path C. When the counter reaches a count,indicative of scan path C receiving the correct number (M/3) SCANCK-Cinputs, it outputs a third count complete 1 (CC1) signal 732 to statemachine 702.

In response to the third CC1 signal, the state machine disables allbuffers 512-516 via signals 718-722 and enables gates 706-710 to passthe SCANCK to all scan cells of scan paths A, B, and C. Since scan pathsA, B, and C were assumed to contain equal numbers of scan cells (M/3)with the sum of the scan cells in scan paths A, B, and C being equal tothe number of scan cells (M) in scan path 104, the third CC 1 signaloccurs one SCANCK prior to controller 110 setting the SCANENA signalhigh, at time 814, during its transition from the operate state 304 tothe capture state 306 in FIG. 6. While SCANENA is high, at time 808, allscan paths A, B, and C receive a SCANCK, causing them to load responsedata from logic 108 of FIG. 5. Following the response data loadoperation at time 808, SCANENA from controller 110 returns low at time812 and the above described sequence of separately accessing scan pathsA, B, and C repeats until the test completes and controller 110transitions back to idle state 302 of FIG. 6.

Contrasting the scan timing diagrams of FIGS. 4 and 8, it is seen thatcontroller 110 provides the same SCANENA timing for both diagrams. Forexample, (1) the SCANENA high to low transition at time 406 in FIG. 4 isthe same SCANENA high to low transition at time 812 in FIG. 8, (2) theSCANENA low to high transition at time 408 in FIG. 4 is the same SCANENAlow to high transition at time 814 in FIG. 8, (3) the same number ofSCANCKs occur between time 406/812 and time 408/814 in both diagrams,and (4) the same response load SCANCK occurs at time 404 in FIG. 4 andat time 808 in FIG. 8. The difference between the two timing diagrams isseen in the way the adaptor 504 sequentially applies a burst of M/3SCANCKs to scan paths A, B, and C during time intervals 802, 804, and806, respectively, such that only one of the scan paths is accessed at atime.

While the example adaptor circuit of FIG. 7 has been described using agated clocking scheme to control access to the scan cells 200 of scanpaths A, B, and C, other example designs of adaptor 504 may be used tocontrol access to other types of scan cells used in scan paths A, B, andC as well. For example, the scan cells 200 of FIG. 2 could be designedto include a state hold multiplexer 218 between the output ofmultiplexer 202 and input to D-FF 204. The state hold multiplexer 218could be controlled, via a connection 220 to the ENACK-A 724, ENACK-B726, and ENACK-C 728 signals from state machine 702, such that itprovides a connection 222 between the output of multiplexer 202 and theD-FF input, or it provides a state hold connection 224 between theoutput of DFF 204 and the input to D-FF 204. If this type of scan cell200 were used in scan paths A, B, and C, the SCANCK 212 could bedirectly routed to all the D-FF 204 clock inputs instead of being gatedto the D-FF 204 clock inputs via the SCANCK-A, SCANCK-B, and SCANCK-Csignals as described for adaptor 504 of FIG. 7. The adaptor 504 would bemodified to operate the state holding scan cells by eliminating thegates 706-710 and the SCANCK-A, SCANCK-B, and SCANCK-C outputs, andproviding as outputs the ENACK-A 724, ENACK-B 726, and ENACK-C 728signals from state machine 702. The ENACK-A output would be connected ascontrol input 220 to the state hold multiplexers 218 in the scan cellsof scan path A. The ENACK-B output would be connected as control input220 to the state hold multiplexers 218 in the scan cells of scan path B.The ENACK-C output would be connected as control input 220 to the statehold multiplexers 218 in the scan cells of scan path C.

During functional and response capture operations, the ENACK-A, ENACK-B,and ENACK-C outputs from the modified adaptor 504 would be set to enablea connection between the response signal 206 and input to D-FF 204 ofeach scan cell, via multiplexer 202 and the state hold multiplexer 218.During scan operations to scan path A (timing interval 802), the ENACK-Band ENACK-C outputs would be set to place the scan cells of scan paths Band C in their state hold connection configuration, and ENACK-A would beset to form a connection between the scan input 208 and input to D-FF204 of the scan cells in scan paths A, to allow scan access of scan pathA. During scan operations to scan path B (timing interval 804), theENACK-A and ENACK-C outputs would be set to place the scan cells of scanpaths A and C in their state hold connection configuration, and ENACK-Bwould be set to form a connection between the scan input 208 and inputto D-FF 204 of the scan cells in scan paths B, to allow scan access ofscan path B. During scan operations to scan path C (timing interval806), the ENACK-A and ENACK-B outputs would be set to place the scancells of scan paths A and B in their state hold connectionconfiguration, and ENACK-C would be set to form a connection between thescan input 208 and input to D-FF 204 of the scan cells in scan paths C,to allow scan access of scan path C.

The modified adaptor 504 and state hold type scan cells described aboveoperate to achieve the low power mode of scan access to scan paths A, B,and C as previously described with the original adaptor 504 and scancell 200. The difference between the two adaptor/scan cell combinationsdescribed above is that the original adaptor/scan cell combinationoperates in a gated clock mode (i.e. uses gated clocks SCANCK-A,SCANCK-B, and SCANCK-C) and the modified adaptor/scan cell combinationoperates in a synchronous clock mode C (i.e. uses the SCANCK).

Scan Path Adaptation

As mentioned previously, test synthesis tools exist that are capable ofautomatically instantiating Scan-BIST architectures similar to the oneshown in FIG. 1. These tools are capable of analyzing logic 108 and itsstimulus and response interface to scan path 104 to determine; (1) whatstimulus data needs to be produced by generator 102 and applied to logic108 via scan path 104, (2) what test signature is expected to beobtained by compactor 106 from the response output from scan path 106,and (3) what type of controller 110 is required to orchestrate thecommunication of stimulus data to and response data from logic 108 viascan path 104. From the analysis, the tool creates the appropriatecontroller 110, generator 102, and compactor 106 circuits and connectsthem to the scan path 104 as seen in FIG. 1. To reduce the effortrequired to adapt the synthesized Scan-BIST architecture of FIG. 1 intothe low power Scan-BIST architecture of FIG. 5, the scan path adaptationprocess described below is preferably performed.

In FIG. 9, scan path 104 is shown receiving stimulus frames 920 fromgenerator 102 via connection 118 and outputting response frames 922 tocompactor 106 via connection 120. The term “frame” simply indicates thenumber of scan bits (M) required to fill the scan path 104 with stimulusdata from generator 102 and empty the scan path 104 of response data tocompactor 106 during the operate state 304 of FIG. 3. The test mayrequire a large number of stimulus and response frame communications totest logic 108. To achieve the low power mode of operation of thepresent disclosure, it is desired to reorganize scan path 104 into aplurality of separate scan paths. In this example, the reorganization ofscan path 104 results in the previously described scan path 502, whichcontains three separate scan paths 506-510. It is also desired to adaptscan path 104 into scan path 502 in such a way as to avoid having tomake any modifications to the synthesized generator 102, compactor 106,or controller 110.

As previously mentioned in regard to FIG. 5, the number (M) of scancells in scan path 104, is assumed divisible by three such that scanpath 104 can be seen to comprise three separate scan segments A, B, andC, each scan segment containing a third (M/3) of the scan cells (M) inscan path 104. Scan segment A of 104 contains a subset 912 of thestimulus and response signals of the overall stimulus and responsebusses 122 and 124 respectively. Scan segment B of 104 contains a subset910 of the stimulus and response signals of the overall stimulus andresponse busses 122 and 124 respectively. Scan segment C of 104 containsa subset 912 of the stimulus and response signals of the overallstimulus and response busses 122 and 124 respectively.

Each stimulus scan frame 920 scanned into scan path 104 from generator102 can be viewed as having bit position fields [CBA] that fill scansegments C, B, and A, respectively. For example, following a scanoperation, bit position field A is loaded into segment A, bit positionfield B is loaded into segment B, and bit position field C is loadedinto segment C. Likewise, each response scan frame 922 scanned from scanpath 104 to compactor 106 can be viewed as having bit position fields[CBA] that empty scan segments C, B, and A, respectively. For example,following a scan operation, bit position field A is unloaded fromsegment A, bit position field B is unloaded from segment B, and bitposition field C is unloaded from segment C. To insure that the stimulus920 and response 922 frames from generator 102 and to compactor 106,respectively, are reusable when scan path 104 is reorganized into thelow power configuration, the reorganization process occurs as describedbelow.

Scan path 104 segment A is configured as a separate scan path A 506, asindicated by the dotted line 914. Scan path 104 segment B is configuredas a separate scan path B 508, as indicated by the dotted line 916. Scanpath 104 segment C is configured as a separate scan path C 510, asindicated by the dotted line 918. The scan inputs to scan paths A, B,and C 506-510 are connected to generator 102 via connection 118. Thescan outputs from scan paths A, B, and C 506-510 are connected, via thepreviously described 3-state buffers 512-516, to compactor 106 viaconnection 120. Each separate scan path 506-510 maintains the samestimulus and response bussing connections 908-912 to logic 108.

Operating the reorganized scan path 502 using the same generator 102 andcompactor 106 circuits used to operate scan path 104 results in thefollowing behavior. This behavior assumes adaptor 504 has been insertedbetween the controller 110 and scan path 502, to control scan path 502as described in FIGS. 5, 6, 7, and 8. During input and output ofstimulus and response frames [CBA] 920 and 922 respectively, (1)stimulus bit field A is directly loaded into scan path A from generator102 as response bit field A is directly unloaded from scan path A tocompactor 106, (2) stimulus bit field B is directly loaded into scanpath B from generator 102 as response bit field B is directly unloadedfrom scan path B to compactor 106, and (3) stimulus bit field C isdirectly loaded into scan path C from generator 102 as response bitfield C is directly unloaded from scan path C to compactor 106. As seenfrom this description, when scan path 104 is reorganized into scan path502 as described, scan path 502 can use the same stimulus and responseframes originally intended for use by scan path 104. Thus nomodifications are necessary to the synthesized generator 102, compactor106, or controller 110 circuits.

In the case where scan path 104 contains a number of scan cells (M) thatis not equally divisible by the desired number of separate scan paths(N) in scan path 502, the length of one of the separate scan paths canbe adjusted to compensate scan path 502 for proper input and output ofthe scan frames 920 and 922. For example, if the number of scan cells(M) in scan path 104 is not equally divisible by the number of separatescan paths (N) required to achieve a desired low power mode ofoperation, M can be increased by adding a value (Y) such that M+Y isequally divisible by N. Once this is done, N separate scan paths may beformed. N−1 of the separate scan paths will have a length (M+Y)/N andone of the separate scan paths will have a length of ((M+Y)/N)−Y. Forexample, if scan path 104 had 97 scan cells (M), scan path A and B of502 would each be configured to contain 33 scan cells[(M+Y)/N=(97+2)/3=33], while scan path C would be configured to contain31 scan cells [((M+Y)/N)−Y=((97+2)/3)−2=31]. In this example, the scanframe 920 and 922 [CBA] segments would be seen as; segment A=33 bits,segment B=33 bits, and segment C=31 bits.

When scan path 502 is formed to include the scan frame compensationtechnique described above, the operation of adaptor 504 is adjusted soit can properly control the compensated scan path 502. In FIGS. 7 and 8,the adaptor 504 circuit and operation was described in detail. Assumingthe adaptor timing diagram in FIG. 8 is being used to communicate scanframes to a scan path 502 consisting of the above mentioned 33-bit scanpath A, 33-bit scan path B, and 31-bit scan path C, the followingchanges are required to adaptor 504. Adaptor state machine 702 continuesto monitor the CC1 732 output from counter 704, as previously described,to determine when to stop 33-bit scan operations to scan paths A and Bat timing intervals 802 and 804, respectively, in FIG. 8. However, sincethe scan timing interval 806 to scan path C is different from the scantiming intervals 802 and 804, the state machine operation is altered towhere it monitors the count complete 2 (CC2) output 734 from counter 704to stop the 31-bit scan operation to scan path C. The CC2 734 output isdesigned to indicate when the 31-bit scan operation to scan path Cshould be stopped, whereas the CC1 732 is designed to indicate when the33-bit scan operation to scan paths A and B should be stopped.

Parallel Scan-BIST Architectures

FIG. 10 illustrates circuit 1000 that has been configured for testingusing a conventional parallel Scan-BIST architecture. As with theprevious single Scan-BIST architecture of FIG. 1, parallel Scan-BISTarchitectures may be synthesized and automatically inserted into ICs toserve as embedded testing mechanisms. The parallel Scan-BISTarchitecture includes; generator 1002, compactor 1004, controller 1008,and scan paths 1-N 1010-1016. During functional mode of circuit 1000,the D-FFs 204 of scan paths 1-N are configured to operate with logic1006 to provide the circuit 1000 functionality. During test mode, theD-FFs 204 of scan path 1-N are configured to operate with generator1002, compactor 1004, and controller 1008 to provide testing of logic1006. Scan paths 1-N receive response from logic 1006 via paths1040-1046, and output stimulus to logic 1006 via paths 1048-1054. Scanpaths 1-N receive serial stimulus from generator 1002 via paths1010-1024, and output serial response to compactor 1004 via paths1026-1032. Scan paths 1-N receive control input from controller 1008 viapath 1034, generator 1002 receives control input from controller 1008via path 1038, and compactor 1004 receives control input from controller1008 via path 1036.

When circuit 1000 is first placed in the test configuration of FIG. 10,the parallel Scan-BIST architecture will be in the idle state 1102 ofthe operation diagram 1100 in FIG. 11. In response to a start testsignal, as previously described in regard to FIG. 1, the parallelScan-BIST architecture transitions from the idle state 1102 to theoperate state 1104. In the operate state, controller 1008 outputscontrol to generator 1002, scan paths 1-N, and compactor 1004 to startthe test. During the operate state, scan paths 1-N are filled withstimulus to be input to logic 1006 from generator 1002 and emptied ofresponse from logic 1006 to compactor 1004. After the scan paths 1-N arefilled and emptied, controller 1008 transitions to the capture state1106 to load the next response data, then returns to the operate state1104 to input the next stimulus from generator 1002 and empty the nextresponse to compactor 1004. After all stimulus and response patternshave been applied, by repeating transitions between the operate andcapture states, the test is complete and the controller returns to theidle state 1102.

The structure and operation of the parallel Scan-BIST architecture ofFIG. 10 is very similar to the structure and operation of the singleScan-BIST architecture of FIG. 1. Some of the most notable differencesbetween the Scan-BIST architectures of FIGS. 1 and 10 include. (1) InFIG. 10, multiple parallel scan paths 1-N are formed during the testconfiguration, as opposed to the single scan path 104 formed during theFIG. 1 test configuration. (2) In FIG. 10, generator 1002 outputsmultiple parallel stimulus outputs 1018-1024 to scan paths 1-N, asopposed to generator 102 outputting a single stimulus output 118 to scanpath 104. (3) In FIG. 10, compactor 1004 inputs multiple parallelresponse outputs 1026-1032 from scan paths 1-N, as opposed to compactor106 inputting a single response output 120 from scan path 104.

The parallel Scan-BIST architecture of FIG. 10 suffers from the samepower consumption problem described in the Scan-BIST architecture ofFIG. 1, since during scan operations, logic 1006 receives simultaneousrippling stimulus inputs from scan paths 1-N. Thus, the parallelScan-BIST architecture of FIG. 10 can be improved to where it consumesless power during test by adapting it into a low power parallelScan-BIST architecture as described below.

Low Power Parallel Scan-BIST Architecture

FIG. 12 illustrates the FIG. 10 parallel Scan-BIST architecture after ithas been adapted for low power operation. The adaptation process, aspreviously described in the low power adaptation of the FIG. 1 Scan-BISTarchitecture, involves the following steps. Step one includesreconfiguring scan paths 1-N 1010-1016 of FIG. 10 into scan paths 1-N1202-1208 of FIG. 12, wherein each scan path 1-N 1202-1208 containsmultiple separate scan paths between their respective inputs 1018-1024and outputs 1026-1032. In this example, it is assumed that each scanpath 1-N 1202-1208 has been reconfigured into separate scan paths A, B,and C, as scan path 104 of FIG. 1 was reconfigured into scan path 502 ofFIG. 5. Step two includes inserting adaptor 1210 between controller 1008and scan paths 1-N 1202-1208. In this example, it is assumed thatadaptor 1210 is very similar to adaptor 504 in the way it operates theseparate scan paths A, B, and C in each of the scan paths 1-N 1202-1208,so only the brief operation description of adaptor 1210 is given below.

As seen in the operation diagram of FIG. 13, adaptor 1210 responds tocontroller 1008 entering the operate state 1104 to: (1) simultaneouslyoperate the scan paths A of scan paths 1202-1208, via control bus 1212,to input stimulus from generator 1002 and output response to compactor1004, then (2) simultaneously operate the scan paths B of scan paths1202-1208, via control bus 1212, to input stimulus from generator 1002and output response to compactor 1004, then (3) simultaneously operatethe scan paths C of scan paths 1202-1208, via control bus 1212, to inputstimulus from generator 1002 and output response to compactor 1004.Adaptor 1210 suspends scan operations to scan paths 1202-1208 whencontroller enters the capture state 1106, and resumes the abovedescribed scan operation sequence to the scan paths A, B, and C of scanpaths 1202-1208 when controller re-enters the operate state 1104. Afterthe test completes, controller 1008 enters the idle state 1102 and theadaptor 1210 is disabled. From this description, the operation ofadaptor 1210 is seen to mirror the operation of adaptor 504 with theexception that adaptor 1210 controls multiple scan paths A, multiplescan paths B, and multiple scan paths C during its control state diagramsequence 1302. In contrast, adaptor 504 controlled only one scan path A,one scan path B, and one scan path C during its control state diagramsequence 602.

Direct Synthesis of Low Power Scan-BIST Architectures

While the process of adapting pre-existing Scan-BIST architectures forlow power operation has been described, it is anticipated that, once thelow power benefit of the present disclosure is understood, testsynthesis tools will be improved to provide direct synthesis of lowpower Scan-BIST architectures. Direct synthesis of low power Scan-BISTarchitectures will eliminate the need to perform the adaptation stepspreviously described, since the steps will be incorporated into thesynthesis process. The following examples describe the low powerScan-BIST architecture concepts of the present disclosure as they may beincluded in synthesized low power Scan-BIST architectures of FIGS. 14and 16.

FIG. 14 illustrates an example synthesis of a single scan path low powerScan-BIST architecture. The previously described adaptation step ofreconfiguring scan path 104 into scan path 502 is shown being includedin the synthesis of the Scan-BIST architecture. The previously describedadaptation step of providing control operable to separately access scanpaths A, B, and C of scan path 502 is also shown being included in thesynthesis of the Scan-BIST architecture. The synthesized low powercontroller 1402 integrates the control features of the previouslydescribed controller 110 and adaptor 504 of FIG. 5 into a single controlcircuit. Controller 1402 operates according to the controller statediagram of FIG. 15, which includes an idle state 1502 corresponding toidle state 302 of FIG. 6, operate states 1504-1508 corresponding tooperate states 304 and 604-608 of FIG. 6, and a capture state 1510corresponding to capture state 306 of FIG. 6.

FIG. 16 illustrates an example synthesis of a parallel scan path lowpower Scan-BIST architecture. The previously described adaptation stepof reconfiguring scan paths 1010-1016 into scan paths 1202-1208 is shownbeing included in the synthesis of the Scan-BIST architecture. Thepreviously described adaptation step of providing control operable toseparately access scan paths A, B, and C of scan paths 1202-1208 is alsoshown being included in the synthesis of the Scan-BIST architecture. Thesynthesized low power controller 1602 integrates the control features ofthe previously described controller 1008 and adaptor 1210 of FIG. 12into a single control circuit. Controller 1602 operates according to thecontroller state diagram of FIG. 17, which includes an idle state 1702corresponding to idle state 1102 of FIG. 13, operate states 1704-1708corresponding to operate states 1104 and 1304-1308 of FIG. 13, and acapture state 1710 corresponding to capture state 1106 of FIG. 13.

Scalable Scan-BIST Power Consumption

As can be anticipated from the description given for the presentdisclosure, the power consumption of logic circuitry being tested by thelow power scan-BIST architecture decreases as the number separate scanpaths within the low power scan paths increases. For example,configuring a given conventional scan path into a low power scan pathcomprising two separate scan paths may reduce power consumption by up to50%, since, during operation, each of the two separate scan pathsseparately charge and discharge one half, potentially, of the logiccircuitry capacitance charged and discharged by the convention scanpath. Further, configuring the same conventional scan path into a lowpower scan path comprising three separate scan paths may reduce powerconsumption by up to 66%, since, during operation, each of the threeseparate scan paths separately charge and discharge one third,potentially, of the logic capacitance charged and discharged by theconvention scan path. Still further, configuring the same conventionalscan path into a low power scan path comprising four separate scan pathsmay reduce power consumption by up to 75%, since, during operation, eachof the four separate scan paths separately charge and discharge onefourth, potentially, the logic capacitance charged and discharged by theconvention scan path. From this it is seen that the present disclosureallows a synthesis tool to be provided with the capability of scalingthe power consumption of a given synthesized scan-BIST architecture tomeet a desired low power mode of test operation of a circuit.

Scalable Scan-BIST Noise Reduction

As can be anticipated from the description given for the presentdisclosure, the noise generated by logic circuitry being tested by thelow power scan-BIST architecture decreases as the number separate scanpaths within the low power scan paths increases. For example,configuring a given conventional scan path into a low power scan pathcomprising two separate scan paths may reduce noise generation by up to50%, since, during operation, each of the two separate scan pathsseparately activate only one half, potentially, of the logic circuitryactivated by the conventional scan path. Further, configuring the sameconventional scan path into a low power scan path comprising threeseparate scan paths may reduce noise generation by up to 66%, since,during operation, each of the three separate scan paths separatelyactivate only one third, potentially, of the logic circuitry activatedby the convention scan path. Still further, configuring the sameconventional scan path into a low power scan path comprising fourseparate scan paths may reduce noise generation by up to 75%, since,during operation, each of the four separate scan paths separatelyactivate one fourth, potentially, of the logic circuitry activated bythe convention scan path. From this it is seen that the presentdisclosure allows a synthesis tool to be provided with the capability ofscaling the noise generation of a given synthesized scan-BISTarchitecture to meet a desired low noise mode of test operation of acircuit.

Although the present disclosure has been described in accordance to theembodiments shown in the figures, one of ordinary skill in the art willrecognize there could be variations to these embodiments and thosevariations should be within the spirit and scope of the presentdisclosure. Accordingly, modifications may be made by one ordinarilyskilled in the art without departing from the spirit and scope of theappended claims.

What is claimed is:
 1. A scan circuit comprising: A. a functionalcircuit formed on the semiconductor substrate of an integrated circuit,the functional circuit including logic circuits to be tested; B. scanpath circuitry having plural scan paths, each scan path being formed ofserially connected scan cells, each scan cell including a multiplexerhaving an enable input, a response data input, a scan data input, and anoutput, and a flip-flop having a data input connected to the output ofthe multiplexer, a clock input and an output providing stimulus data andscan data, each scan path having leads connected to the logic circuitsto carry stimulus signals to the logic circuits and to receive responsesignals from the logic circuits, each scan path having a serial datainput lead and a serial data output lead, each scan path being organizedin at least first and second selectable and separate scan path parts,each scan path part having a serial input connected to the serial datainput lead of that scan path, a serial output lead selectively coupledto the serial data output lead of that scan path, and a separate set ofscan path part enable and clock input leads; C. test data generatorcircuitry having control inputs and a serial data output connected tothe serial data input lead of each scan path; D. test data compactorcircuitry having control inputs and a serial data input coupled to theserial data output lead of each scan path; E. controller circuitryhaving control outputs coupled with the control inputs of the testgenerator circuitry, control outputs coupled with the control inputs ofthe test data compactor, and enable and clock outputs for the scanpaths; and F. adaptor circuitry having control inputs connected with theenable and clock outputs for the scan paths of the controller circuitry,the adaptor circuitry having a first set of scan path part enable andclock output leads connected with the separate set of scan path partenable and clock input leads of the first scan path parts and a secondset of scan path part enable and clock output leads connected with theseparate set of scan path part enable and clock input leads of thesecond scan path parts, the adaptor circuits including a state machinecontrolling the production of the separate sets of control signals foroperating the respective separate scan path parts.
 2. The circuit ofclaim 1 in which the scan path parts of each scan path have equalnumbers of scan cells.
 3. The circuit of claim 1 in which the scan pathshave equal numbers of scan cells.